(1) Field of the Invention
The present invention relates to an interoffice hand-off processing system and an interoffice hand-off method, and more particularly, to an interoffice hand-off processing system provided in a mobile communication exchange having an interoffice control signal link and an interoffice speech link for communications with other mobile communication exchanges and to an interoffice hand-off method executed in the processing system.
(2) Description of the Related Art
Generally, in a mobile communication system, when a mobile unit moves from a service zone covered by a base station connected to a first mobile communication exchange to another service zone covered by another base station connected to a second mobile communication exchange during a call, it is necessary that an interoffice hand-off process for maintaining the call of the mobile unit should be performed between the first and second mobile communication exchanges. In a mobile communication system using CDMA (Code Division Multiple Access) technique in particular, soft hand-off is carried out as such hand-off. The hand-off is also called hand-over.
Conventionally, when the interoffice hand-off process is required due to movement of a mobile unit, the mobile communication exchange away from which the mobile unit is moving acquires an idle channel in an interoffice control signal link and, using the acquired idle channel, transmits an interoffice hand-off signal to the mobile communication exchange toward which the mobile unit is moving. Based on the interoffice hand-off signal, an interoffice speech path using an interoffice speech link is established between the receding-side mobile communication exchange and the approaching-side mobile communication exchange. This permits the mobile unit to maintain the call via the approaching-side mobile communication exchange, the interoffice speech path, and the receding-side mobile communication exchange in order.
The interoffice control signal link is a link which connects between mobile communication exchanges, like the interoffice speech link, for transmitting interoffice control signals therethrough. In CDMA mobile communication systems, the interoffice control signal link is defined as interoffice Map link (IMSCCID) in accordance with the ITU-T (International Telecommunication Union-Telecommunication Sector) recommendations. The interoffice speech link includes ISUP link and TUP link. The ISUP link and the TUP link differ from each other in the form of signal used.
If the number of mobile units making interoffice movement becomes large, the traffic of the interoffice control signal link increases, possibly causing congestion. In the CDMA mobile communication exchange, however, the interoffice Map link and the interoffice speech link are managed separately from each other. It is, therefore, not possible to use the interoffice speech link even when the interoffice Map link is congested. As a result, even if the interoffice hand-off process is required, an idle channel of the interoffice Map link cannot be acquired and thus the interoffice hand-off signal cannot be transmitted, bringing about a situation where the call of a mobile unit making interoffice movement is forcibly disconnected.